1. Field of the Invention
This invention relates to the desensitizing of theft detection sensor elements of protected articles, for example, sensor elements in the spines of books and more particularly it concerns novel methods and apparatus for automatically desensitizing sensor elements when the articles on which they are mounted have been authorized for release from a protected area such as a library.
2. Description of the Related Art
In an effort to save on personnel costs in libraries, various library services have been automated. One such automated service is that of automatic book checkout. Examples of automatic book checkout machines are shown and described in U.S. Pat. Nos. 4,141,078 and 4,881,061. These devices comprise optical sensors which read an identifying code, such as a bar code, on a book to be checked out as well as an identifying code on the library card of a patron checking out the book. After these codes have been read, the device operates a sensor deactivator which deactivates a theft detection sensor on the book. At this point the book may be removed from the device and taken out of the library without triggering an alarm.
A problem encountered in connection with these automatic book checkout devices is that the patron may place several books in the device at the same time. The device will read only the bar code of the topmost book; but when the desensitizer is actuated, it will desensitize all the books in the checkout device. To overcome this problem U.S. Pat. No. 4,141,078 proposes to provide a slanted shelf on which to lay the book for checkout. If two or more books are placed on the shelf, the upper book or books will slide off from the lower one and cause a photoelectric sensor to indicate an improper use of the machine. U.S. Pat. No. 4,881,061 proposes an alternate multiple book sensing technique, namely to measure the thickness of the book using an ultrasonic ranging device. This thickness is compared with the thickness identified by the identifying code on the book; and if the two do not correspond, the desensitizer apparatus is not activated. U.S. Pat. No. 4,881,061 also suggests the possibility of measuring the weight rather than the thickness of the book.
Each of these techniques has inherent disadvantages. Specifically, where a slanted shelf is used, the system must depend on the frictional characteristics of the covers of the books and they must incorporate a complicated system of photoelectric sensors. Also, where the thickness or weight of the book is read from its identifying label, each book's identifying label must be specially coded with the book's thickness or weight. This imposes a substantial burden of special coding individual books, particularly where a large number of books is to be maintained in the library.